This invention relates to ultrasonic transducers and, more particularly, to an ultrasonic transducer with a center frequency that can be dynamically selected in response to a suitable control signal applied to the transducer.
Ultrasonic transducers for medical or industrial applications are generally constructed from one or more piezoelectric elements sandwiched between a pair of electrodes. Such piezoelectric elements are typically constructed of lead zirconate titanate (PZT), polyvinylidene difluoride (PVDF), or PZT ceramic/polymer composite. The electrodes are connected to a voltage source, and when a voltage is applied, the piezoelectric elements are excited at a frequency corresponding to that of the applied voltage. When a voltage pulse is applied, the piezoelectric element emits an ultrasonic beam into the media to which it is coupled at the frequencies contained in the excitation pulse. Conversely, when an ultrasonic beam strikes the piezoelectric element, the element produces a corresponding voltage across its electrodes. Typically, the front of the element is covered with an acoustic beam matching layer that improves the coupling with the media in which the ultrasonic beams propagate. In addition, a backing material is disposed to the rear of the piezoelectric element to absorb ultrasonic beams that emerge from the back side of the element so that they do not interfere. A number of such ultrasonic transducer constructions are disclosed in U.S. Pat. Nos. 4,217,684, 4,425,525, 4,441,503 and 4,470,305, all of which are assigned to the instant assignee.
Typically the transducer can be designed based upon its constituent piezoelectric material and specific construction to operate at a generally fixed predetermined center frequency depending on the particular industrial or medical application chosen for a given transducer. For example, in a given medium, to provide increased depth of penetration for the ultrasonic beam transmitted by the transducer usually requires a corresponding increase to the center frequency. Typical transducers in general provide a frequency response which is relatively confined to near the operating center frequency of the transducer. It is desirable to provide center frequency selectivity in order to, for example, utilize a given individual ultrasonic transducer in a broader array of applications than would be possible if the transducer was designed to operate at a generally fixed center frequency. Although such center frequency selectivity is desirable, operation of an ultrasonic transducer at different center frequencies can be difficult being that certain acoustical properties, such as the modulus of elasticity, of the constituent piezoelectric material of the transducer, which in turn determine the center frequency, are essentially fixed for a given piezoelectric material. In general, schemes proposed heretofore have failed to provide a workable scheme for substantially changing such piezoelectric modulus in a controllable manner. Thus, there is a need in the art to provide an improved ultrasonic transducer which is capable of dynamically and selectably changing its center frequency in a manner that advantageously allows such transducer to expand its range of operation and thus results in a transducer with broad versatility of use in medical and/or industrial applications.